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Equilibrium, PDF Notes, Important Questions and Formulas


Basically, the term refers to what we might call a “balance of forces”. In the case of mechanical equilibrium, this is its literal definition. A book sitting on a table top remains at rest because the downward force exerted by the earth’s gravity acting on the book’s mass is exactly balanced by the repulsive force between atoms that prevents two objects from simultaneously occupying the same space, acting in this case between the table surface and the book. If you pick up the book and raise it above the table top, the additional upward force exerted by your arm destroys the state of equilibrium as the book moves upward. If you wish to hold the book at rest above the table, you adjust the upward force to exactly balance the weight of the book, thus restoring equilibrium. An object is in a state of mechanical equilibrium when it is either static or in a state of unchanging motion.

From the relation f=ma, it is apparent that if the net force on the object is zero, its acceleration must also be zero, so if we can see that an object is not undergoing a change in its motion, we know that it is in mechanical equilibrium.

Another kind of equilibrium we all experience is thermal equilibrium. When two objects are brought into contact, heat will flow from the warmer object to the cooler one until their temperatures become identical. Thermal equilibrium is a “balance of forces” in the sense that temperature is a measure of the tendency of an object to lose thermal energy. A metallic object at room temperature will feel cool to your hand when you first pick it up because the thermal sensors in your skin detect a flow of heat from your hand into the metal, but as the metal approaches the temperature of your hand, this sensation diminishes. The time it takes to achieve thermal equilibrium depends on how readily heat is conducted within and between the objects; thus a wooden object will feel warmer than a metallic object even if both are at room temperature because wood is a relatively poor thermal conductor.

Chemical Equilibrium

When a chemical reaction e.g. 2HI(g) H2(g)+I2(g) takes place in a closed container the quantities of components change as some are consumed and others are formed. Eventually this change will come to an end, after which the composition will remain unchanged as long as the system remains undisturbed. The system is then said to be in its equilibrium state, or more simply, “at equilibrium”.

It makes no difference whether we start with two moles of HI or one mole each of H2 and I2; once the reaction has run to completion, the quantities of these two components will be the same. In general, then, we can say that the composition of a chemical reaction system will tend to change in a direction that brings it closer to its equilibrium composition.

Dynamic equilibrium characteristics:

  • The state at which concentrations of reactants or products do not change with time.
  • It is attained when rate of forward reaction becomes equal to rate of backward reaction.
  • A dynamic equilibrium, attained from either side.

Chemical Equilibrium with Graph :

Reversible Reaction

A chemical equation of the form A → B represents the transformation of A into B, but it does not imply that all of the reactants will be converted into products, or that the reverse reaction B → A cannot also occur. In general, both processes can be expected to occur, resulting in an equilibrium mixture containing all of the components of the reaction system. If the equilibrium state is one in which significant quantities of both reactants and products are present then the reaction is said to incomplete or reversible. In principle, all chemical reactions are reversible, but this reversibility may not be observable if the fraction of products in the equilibrium mixture is very small, or if the reverse reaction is kinetically inhibited.

• Irreversible reactions:

  1. Unidirectional
  2. Goes for completion
  3. No equilibrium is attained
  4. A reaction is said to be irreversible when either of the product is settled down as solid or escapes out as gas, e.g.,

BaCl2+Na2SO4→BaSO4 ↓+ 2NaCl

CaCO3(s) → CaO(s) + CO2(g) + CO2 ↑

• Reversible reactions:

  1. Both direction
  2. Never goes for completion
  3. Attains equilibrium
  4. Otherwise the reaction is reversible e.g.,

CaCO3(s)⇌CaO(s) +CO2(g)

(in closed container)

Homogeneous Equlibrium:

The system in which all the reactant and product have same physical state.





Heterogeneous Equilibrium:

The system in which at least one reactant or product have different physical states from others. e.g.

CaCO3(s)⇌ CaO(s) +CO2(g)

This is three phase system

Salt + H2O(aq) → Salt(aq), Three phase system.

Ionic Equilibrium 

  1. Electric Conductivity: 
    Those substances which allow the electric current to pass through them are called electric conductors and property is called electric conductivity. On the basis of Electric conductivity, substances are of two types -
  2. Non-conductors: 
    Those substance which do not allow the electric current to pass through them are called non-conductors. eg. All covalent compounds & non-metals.
  3. Conductors:
    Those substances which allow the electric current to pass through them are called conductors. For eg. All metals, alloys, all acid and bases, salt and graphite etc. On the basis of conducting units conductors are of two types -
  4. Metallic or Electric Conductors:
    Electricity conducts them due to the presence of free and mobile electron which act as electricity conducting unit called metallic or electric conductors. e.g. Metals, Alloys, Graphite, Gas, Carbon etc.
  5. Ionic Conductors or Electrolytes: 
    Conductors in which the current is passes through them due to the presence of free ions are called Ionic Conductors or Electrolyte or Electrolytic conductors. Ionic conductors are further divided into two types on the basis of their strengths -
    (a) Strong electrolytes:
    (i)       Those substances which are almost completely ionize into ions in their aqueous solution are called strong electrolytes.
    (ii)     Degree of ionisation which are almost completely ionize into ions in their aqueous solution are called electrolytes.
    i.e. ∝ ≅ 1. Eg. HCl, H2SO4, Nacl.NHO3, KOH, NaOH, HNO3, AgNO3, CuSO4, etc. Means all strong acids and bases and all types of salts.
    (b) Weak electroytes:
    (i)    Those substance which are ionize to a small extend in their aqueous solution        are known weak electrolytes.eg. H2O, CH3COOH, NH4OH, HCN, HCOOH, Liq.SO2 etc.
    Means all weak acids and bases
    (ii)  Degree of ionization for this types of electroytes in ∝ <<< 1.

 Concept of electrolyte


                                             Strong electrolyte

                                         Weak electrolyte

A+B-→A+ + B-                                  AB⇌A+ B-

a       0        0                              a     0    0

0       a        a                       a(1-∝)    a∝   a∝

100% dissociation (∝=1)                       ∝ < 1

No equilibrium                                   equilibrium

It is irreversible                                 It is reversible process.


Bronsted and lowry concept of acids & bases postulates :-

(1)  Acid – proton (H+) donor

(2)  Base – Proton (H+) acceptor e.g.

HCl(aq) +H2O(𝑙)⇌H3O+(aq)+Cl-(aq)

Acid          Base

HCl(aq)+NH3(aq)⇌NH4+(aq) +Cl-(aq)

Acid           Base

HCl(aq) + CH3COOH(aq)⇌CH3COOH+(aq)+Cl-(aq)

Note:-Here CH3COOH has a less tendency to donate H+ than HCltherefore CH3COOH acts as a weak base.

Conjugate Acid-Base pair(CABP)

In an acid-base reaction

Acid→H++ conjugate base

Base+ H+ → conjugate acid


HCl(aq) + NH2(aq) ⇌ NH4+(aq)+Cl-(aq)

Note:- A CABP is different from each other only by single proton.


HCl (aq) + NH3(aq)⇌NH3+(aq) +Cl-(aq)

Relative strength of Acids/Bases:-

Any Spices and its conjugate species are opposite of each other in terms of strength e.g.

           Acid (or Base)       Conjugate Base (or Acid)

  1.      Weak                 Strong
  2.      Strong                Weak


Strength order of acids.


Strength order of conjugate bases

ClO4- <HSO4-<Cl-<CH3COO-

An ionic Equilibrium exists between the unionised electrolyte molecules and the ions that result from ionisation

         A + B ⇌ A+ + B-

Arrhenius theory of Electrolytic Dissociation or Ionization

  1. When an electrolyte dissociates into water, it gives two types of charged particles called ions.
  2. Ions which carry (+) ve charge and move towards cathode are called as ‘Cations’ while ions carrying (–) ve charge and moving towards anode called as ‘anion’.
  3. Every electrolytic solution is always neutral in nature.
  4. Quantity or part of electrolyte which is ionized or decomposed or dissociate called as “Degree of Ionisation”.
  5. Electrolyte which gives H+ ions after dissociation in the aqueous solution is called as acid while that which gives OH– after dissociation in the aqueous solution is called as base.
  6. Acidic strength of acids is directly proportional to the dissociation constant Ka.
    a. pKa=-log Ka
    b. Thus, Acidic strength ∝ Ka ∝ begin mathsize 12px style 1 over pK subscript straight a end style ∝  pKb   begin mathsize 12px style 1 over straight K subscript straight b end style
  7. Similarly basic strength of bases is directly proportional to Kb
    i.      pKb =-log Kb
    ii.      Basic strength of base ∝ Kb ∝ begin mathsize 12px style 1 over pK subscript straight b end style∝ pKa begin mathsize 12px style 1 over straight K subscript straight a end style
  8. Conductivity of solution depends upon the number of ions produced by the electroytes, such as-
    Conduction of solution is directly proportional to number of ions produced by the electrolyte
    (i)    Solution of strong electrolyte has more electric conductivity property as compared to weak electrolyte.
    (ii)  Only weak electrolyte followed the law of mass action and Ostwalds dilution law.
  9. When electricity passed through in the electrolytic solution, it gives        only direction to movement of ions towards the electrodes.
  10. Movement of ions is inversely proportional to the molecular mass or atomic mass of ions.

Limitations of Arrhenius Concept:-

  1. H+ and OH- ions exist as hydrated ions.
  2. He was unable to explain the acidic nature of CO2, SO2 etc. and basic nature of NH3,CaO, Na2CO3 etc.
  3. He could not explain the acid base reaction in the absence of water.
    SO3(g) +CaO(s) → CaSO4(g)

Factors affecting the degree of ionisation:

  1. Temperature:- With the rise in temperature, the degree of dissociation of an electrolyte in solution increased.
    Thus, Degree of dissociation ∝ Temperature
  2. Dilution:- On the increasing of dilution, the degree of dissociation increases. But at infinite dilution, there is no effect on the degree of dissociation.
  3. Concentration of the solution: -

    begin mathsize 12px style table attributes columnalign left end attributes row cell Degree text  of dissociation      end text proportional to text      end text fraction numerator 1 over denominator Concentrat text   end text ion text  of solution end text end fraction text   end text end cell row cell proportional to text   end text fraction numerator 1 over denominator table attributes columnalign left end attributes row cell Amount text  of solute in given  end text end cell row cell text Volume or wt.of solution end text end cell end table end fraction text   end text proportional to text  Amount of solvent end text end cell end table end style 
  4. Nature of solvent: - Higher the dielectric constant of a solvent, more is its dissociation power or ionising power.
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